VALIDATION OF A STEAM BASED POST-PROCESS PASTEURIZATION SYSTEM FOR CONTROL OF LISTERIA MONOCYTOGENES IN READY-TO-EAT ROAST BEEF

Listeria monocytogenes has been implicated in outbreaks of illness involving ready-to-eat (RTE) meat products, prompting researchers to look into intervention technologies to reduce or eliminate this risk. In our study roast beef was inoculated with a 5-strain cocktail of Listeria monocytogenes, vacuum-packaged, and then pasteurized at 205°F for 0, 2, 3 or 4 min in a Stork RMS-Protecon Post-Process Pasteurization System. More bacteria were killed as pasteurization time increased. Initial inoculum level was 5.8 log10 CFU/cm of product surface area. Pasteurization for 2 min resulted in 2.5 to 2.7 log10 CFU/cm reductions. Similar reductions were seen at 3 min. At 4 min pasteurization, L. monocytogenes decreased in roast beef by approximately 4.5 log10 CFU/cm; over 99.99% had been killed. The Stork steam based system is effective for reducing the risks of L. monocytogenes in RTE roast beef while providing acceptable quality characteristics.


VALIDATION OF A STEAM BASED POST-PROCESS PASTEURIZATION SYSTEM FOR CONTROL OF LISTERIA MONOCYTOGENES IN READY-TO-EAT ROAST BEEF
V. S. Gill, H. Thippareddi, R. K. Phebus, J. L. Marsden, and C. L. Kastner

Summary
Listeria monocytogenes has been implicated in outbreaks of illness involving ready-to-eat (RTE) meat products, prompting researchers to look into intervention technologies to reduce or eliminate this risk.In our study roast beef was inoculated with a 5-strain cocktail of Listeria monocytogenes, vacuum-packaged, and then pasteurized at 205°F for 0, 2, 3 or 4 min in a Stork RMS-Protecon Post-Process Pasteurization System.More bacteria were killed as pasteurization time increased.Initial inoculum level was 5.8 log 10 CFU/cm 2 of product surface area.Pasteurization for 2 min resulted in 2.5 to 2.7 log 10 CFU/cm 2 reductions.Similar reductions were seen at 3 min.At 4 min pasteurization, L. monocytogenes decreased in roast beef by approximately 4.5 log 10 CFU/cm 2 ; over 99.99% had been killed.The Stork steam based system is effective for reducing the risks of L. monocytogenes in RTE roast beef while providing acceptable quality characteristics.

Introduction
Listeria monocytogenes can contaminate cooked meat products between cooking and final packaging.Therefore, research is being focused on pasteurization of surfaces of cooked, ready-to-eat (RTE) meat products after packaging.
Both irradiation and thermal based pasteurization systems are being investigated.However, thermal based methods are easiest to implement because they do not require regulatory approval.Thermal systems, those based on saturated steam, appear to achieve the surface temperatures required to destroy L. monocytogenes in 1 to 4 min, without significantly affecting the sensory quality of the RTE meat products.
Our objective was to evaluate the effectiveness of the Stork RMS-Protecon Post-Process Pasteurization System in reducing or eliminating L. monocytogenes on RTE meat product surfaces after final packaging.

Experimental Procedures
Pre-cooked beef roasts (ca. 10 lb each) were stored at 40°F until pasteurization.Immediately before inoculation, beef roasts were cut into two equal halves.The resulting fresh-cut surfaces were labeled as "bottom" sides of the meat.The bottom sides were placed down to replicate retail conditions.
Five strains of L. monocytogenes were mixed with sterile 0.1% peptone water to achieve a final concentration of ca. 1 × 10 9 CFU/ml.
Beef roasts were placed on sterile stainless steel wire racks resting on a stainless steel trough.The inoculum was sprayed on both top and bottom surfaces by "misting" the mixed-strain inoculum in a "bio-containment" chamber.After inocu-lation the products were placed in a laminar flow cabinet for one hour at room temperature to allow microbial attachment.Then all products were vacuum-packaged in CNP 320 cook-in bags (Cryovac, Duncan, SC) and pasteurized at the Kansas State University Aseptic Processing Laboratory.
All inoculated beef roasts except unpasteurized (0 min) were surface pasteurized in the Stork RMS-Protecon Post-Processing Pasteurization System at 205°F for 2, 3 or 4 min.Immediately after pasteurization, products were immersed in an ice-water bath for 10 min before sampling.
Samples were taken from the top and bottom surfaces.The flat bottom surfaces were cored twice (7.36 in 2 ) and top surfaces were cored three times (11.04 in 2 ).Cores from each sampling location per product were combined with 0.1% sterile PW, serially diluted, plated on Modified Oxoid Agar MOX (Difco Laboratories, Detroit, MI) and incubated at 100°F for 24 h.Colonies were counted and reported as log 10 CFU/cm 2 .There were four replications for each treatment.

Results and Discussion
As pasteurization time increased, Listeria recovery decreased proportionally.Pasteurization for 2, 3, and 4 min resulted in reductions of 2.67, 3.57, and 4.48 log 10 CFU/cm 2 on the top portion from an initial level of 5.98 log 10 CFU/cm 2 , and reduction of 2.51, 2.34, and 2.53 log 10 CFU/cm 2 on the bottom portion from an initial level of 5.82 log 10 CFU/cm 2 .On the top surface, 3.30 log10 CFU/cm 2 survived 2 min of pasteurization, while 1.49 log 10 CFU/cm 2 survived pasteurization for 4 min (P≤0.05).More L. monocytogenes survived 3 or 4 min of pasteurization on the bottom than the top surface (P≤0.05).Reducing a microbial population of 5 log 10 CFU/cm 2 down to 2 log 10 represents a reduction of 99.9%.the microbial population added to our samples through inoculation are much higher than would be seen under naturallyoccurring circumstances.
The USDA-FSIS mandates a zero tolerance policy for L. monocytogenes (FSIS, 1989) in RTE meat and poultry products.The mortality rate for L. monocytogenes infection is about 33% because of the virulence of this organism.Exposure to even a single cell may cause septicemia, meningitis, and abortion in susceptible humans.Earlier studies have reported about 53% of samples testing positive for L. monocytogenes in vacuum packaged RTE sliced meat.
This study validates the effectiveness of a saturated steam-based post-processing system for reducing or eliminating L. monocytogenes on surfaces of RTE deli meat products.The Stork RMS-Protecon system can be integrated into the RTE meat product manufacturing process as a Critical Control Point to reduce or eliminate the risk of Listeria monocytogenes in these products.